Cellular phone handsets are required to set transmit power to within a specified precision. There are two predominant techniques. The first is the in factory calibration performed when the handset is being manufactured. In calibration, the handset is measured to ascertain the output power under various circumstances, and a table of the results is generated and stored within the handset. This table is used to set the power per the direction of the system. The accuracy of the power setting is then determined by how thoroughly this calibration is accomplished. This technique is not capable of responding to changes in the performance of the handset.
The second technique is sample and detect. The power out of the transmit portion is sampled and detected. The second technique requires a coupler, detector, and signal processing to measure the detected voltage as will be further described. This requires that a form of calibration be performed, but the detection circuit will accurately reflect any subsequent changes in the performance of the handset.
FIG. 1 schematically illustrates how a coupler works. Any two conductors, e.g. transmission lines, sufficiently near one another will function as a coupler. Power delivered into a first transmission line will couple into a parallel second transmission line, and flow in a direction opposite to that in the first transmission line. The amount of coupling is a function of the separation between the two transmission lines and the multiple of wavelengths that the separation embodies.
FIG. 2 illustrates a dual directional coupler. The coupler can detect both incident and reflected power.
Using either prior art coupler, the detected power is then delivered to a detector diode. The diode rectifies the power and generates a DC level. This DC level is processed according to the system needs. The detected value is used to adjust the power level as required.
The process technology used to implement the coupler sets the minimum separation between the through conductor, e.g. first transmission line, and the coupled conductor, e.g. second transmission line. This minimum separation determines the minimum length to achieve the desired coupling. To illustrate, driving a diode directly requires about 15 dBm at 1 to 2 GHz, the range of interest for handsets. If the amplifier is transmitting 1 W (30 dBm), then the coupler must provide 15 dB of coupling. This requirement sets the minimum length of the coupler in any particular process technology.
There are two loss mechanisms in a coupler. The first is the ideal loss associated with the coupled power. This power leaves the through path and enters the coupled path. When half the power is coupled in a 3 dB, the through loss is at least 3 dB. In a 15 dB coupler, the through loss is at least 0.14 dB.
The second loss mechanism is resistive. The metals and dielectrics used in a coupler are inherently lossy. Consequently, the longer the through transmission line is the higher the loss. FIG. 3 shows the ideal coupler loss vs. coupling for a commercially available ceramic coupler supplied by AVX Inc.
Couplers are available in many form factors. The largest are instrument grade, made of machined metal, operable over many octaves. The smallest are built on ceramic, covering perhaps one octave usefully, e.g. small ceramic AVX 15 dB coupler having 0.35 dB loss at 2 GHz. To implement the detector function, the circuit includes the ceramic coupler, external diodes, a biasing network for the diodes, bypass capacitors, and terminating resistors, if needed. The resulting network is large and unwieldy.